Membrane separation refers to a method for separation, concentration and purification of a raw material by using a selective permeation membrane, in which the components of the raw material at the side of raw material selectively permeate the membrane when there exists a certain driving force (such as pressure difference, concentration difference, potential difference or temperature difference etc.). Different membranes and driving forces are employed in different membrane separation processes. At present, the membrane separation processes that have been industrially used include microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), dialysis (D), electrodialysis (ED), gas separation (GS), pervaporation (PV) and emulsion liquid membrane (ELM) etc. In addition, there are many novel membrane separation processes under development, such as membrane extraction, membrane distillation, bipolar membrane electrodialysis, membrane split phase, membrane absorption, membrane reaction, membrane control release, membrane biosensor, etc.
As compared to traditional separation methods, the membrane separation technique has the following advantages: (1) High efficiency: since a membrane is selective, some substances can pass through it selectively, while other substances are retained by it. Effective separation, purification and concentration can be performed by selecting and utilizing a suitable membrane; (2) Energy saving: most of membrane separation processes are operated at a room temperature without phase transition of a separated substance, so membrane separation technique is an unit operation with low energy consumption and low cost; (3) Membrane separation processes are simple, easy to be operated and controlled; and (4) There is no environmental pollution during the membrane separation processes.
Therefore, the membrane separation technique has been developed rapidly in recent years, widely applied in petrochemical industry, biological pharmaceutical industry, medical and sanitation fields, metallurgy industry, electronics, energy field, light industry, textile industry, food industry, environmental protection industry, aerospace industry, maritime transport industry and daily life field, and becomes one of the most important means in separation science nowadays.
However, concentration polarization phenomenon generally existing in membrane separation processes is one of main factors affecting membrane flux and causing membrane fouling. Concentration polarization phenomenon refers to a phenomenon that a separation membrane selectively allows some components in a raw material to be separated to pass through but other components to be retained, which results in the enriching of the retention components near to the membrane surface of separation side to form a concentration gradient from the membrane surface to the raw material bulk phase, thereby causing a diffusion of the retained components from the membrane surface to the raw material bulk phase and a decrease of membrane flux. For example, during membrane separation of a solution, the treated solution convectively flows to membrane surface, and the retained solute accumulates near to the membrane surface, so that the concentration of solute on membrane surface is higher than that in the solution bulk phase, and a concentration gradient from the membrane surface to solution bulk phase is formed, which causes a diffusion of the retained components from the membrane surface to the raw material bulk phase and a decrease of flux. The above phenomenon is called concentration polarization.
Since concentration polarization not only causes the decrease in membrane flux but also aggravates membrane fouling due to the enriching of retention components on membrane surface, concentration polarization is a problem generally to be solved during membrane separation processes. For example, the substances with a high concentration on membrane surface may be removed by tangential flow based on the optimization of membrane module design and of operation conditions to reduce concentration polarization on membrane surface and to maintain membrane flux. However, the methods and equipments for reducing concentration polarization and membrane fouling to maintain membrane flux still need to be developed.